[Aroy]

Plus Side

The diesel engine is inherently more efficient than a petrol engine. So it would use less fuel to deliver same power.

Due to higher pressure and temparature in a diesel engine, it is built more robust (hence more weight) compared to a petrol engine, so last longer. Modern automobile diesels have evolved a lot and high strength light alloys are now used to reduce the dead weight, while keeping long life.

Turbocharging is common and generates at least 50% more power. The side effect is that diesels with turbo can go to high altitudes, where petrol engines suffer due to rarefied air,

Diesels were noisier and had more vibration than petrols. This was due to uncontrolled ignition. With computer controlled fuel injection, the loud noisy and heavily vibrating diesels are thing of past, and a well designed diesel in a car can rarely be distinguished from its petrol counterpart.

Diesels have lot more low end torque. This translates into excellent power at low speeds, ideal for slow city traffic. Where you may have to change two or three gears in a petrol car, you can do with practically no gear change.

Minus Side

Normal automotive diesel engine has lower RPM range compared to its petrol counterpart. This means that it would require more over drive gears for the vehicle to go as fast as a normally geared free revving petrol engine. That is not to say that a diesel engine cannot be high revving - some early aircraft diesel engines could achieve very high RPM, but proved too heavy compared to their petrol counterpart. In fact I recollect that my model aircraft diesel engine had better performance compared to comparable petrol counterparts.

Diesel engines generally are more polluting in terms of particulate matter (basically unburnt carbon particles), though higher efficiency ensures that the CO and other gasses are lower. Better ignition control and use of particulate filters in the exhaust will result in really low polluting engine.

Diesel engines generate more SO2/SO3 gasses in their exhaust, but that is a fuel problem and not engine problem. With better fuel this should be of no consequence. Still fuel is a part and parcel of the vehicle running.

[sam_b]

Technology-wise the diesel engines of today are so much more evolved from those of yesteryears. Having said that, IMHO the quality of refinement that one gets out of a petrol is unchallenged by a diesel engine. Also on busy two lane roads, petrol gives more confidence to overtake (within road rules) than a diesel. I guess, it makes economical sense to go diesel if you travel say > 6-8000 kms a year or else sticking to petrol would be the way to go..

Cheers!
S

[alpha1]

NAVRDDY
This is not Information technology where you absolutely have to come up with a new release every quarter, or your sales go down.
Or the competition will come up with something better ...

Petrol engines have not changed much in the past century.
And there is a reason for it - you cannot change the working principles of the machines.
What you can do is change the fuel injection system, turbo charge or super charge.
Whatever you see about NEW TECHNOLOGY in diesel is because Diesel engines were poor performers compared to the Petrol ones.

THUS, they had to come up with newer technology to bring them at par.

Do you want petrol engine makers to just come up with a new technology every quarter so that ppl can boast about it ... but when it comes to actual working - it delivers inferior results compared to earlier, and needs constant "reboot".

[vina]

Originally Posted by alpha1

NAVRDDY
This is not Information technology where you absolutely have to come up with a new release every quarter, or your sales go down.
Or the competition will come up with something better ...

Petrol engines have not changed much in the past century.
And there is a reason for it - you cannot change the working principles of the machines.
What you can do is change the fuel injection system, turbo charge or super charge.
Whatever you see about NEW TECHNOLOGY in diesel is because Diesel engines were poor performers compared to the Petrol ones.

THUS, they had to come up with newer technology to bring them at par.

Do you want petrol engine makers to just come up with a new technology every quarter so that ppl can boast about it ... but when it comes to actual working - it delivers inferior results compared to earlier, and needs constant "reboot".

Disclaimer: I would not buy a diesel car if petrol and diesel fuels cost the same, simply because the diesel car would still cost much more. If on the other hand the fuels cost the same and the difference in prices of the cars was small enough to be recovered by the diesels' better FE in a few years - I'll have to at least think very hard.

Presently I own a Figo TDCi


Having said that, I would say it is unfair to claim that petrol engines are better than diesels. In fact at a fundamental level, the diesel engines provide better combustion cycles, more flexibility as to what fuel can be used and a lot of other advantages. To my knowledge almost all large ICE engines are diesels - even the ones that use alternative fuel.

Also technology has benefited petrols also immensely: MPFI, VVT etc. help petrols too. In fact no amount of technology will ever help the basic petrol limitation of compression ratio, and a minimum fuel/air ratio.

[dhanushs]

Even now, even though, fuel bills are blowing a hole in my pocket, I'm trying real hard to find reasons NOT to buy/swap to a diesel.

I come home late ~11PM. Traffic is sparse(Nil). Roads are twisty.

For a about a month I was lucky enough to take turns in driving a Fiesta 1.4TDCi, Swift 1.3DDIS and my '07 IKON Flair home. And even though my iKON had the least advanced tech, it was the most fun driving home. I'm just too addictive to the sound of the engine while revving up and while downshifting right to the redline!.

Apart from handling, the 1.4 TDCi Fiesta was dud! The swift did put a grin on my face with the turbo-kick, but nothing compared to revving a petrol!. And if its a VTEC with a slick manual, then let diesels go to H***!!..

Originally Posted by vina

.. To my knowledge almost all large ICE engines are diesels - even the ones that use petrol fuel...

Umm.. Is that a typo?.

[vina]

Originally Posted by dhanushs

...

Umm.. Is that a typo?.

That's 2AM in the morning changing words as one types. It was supposed to be "alternative fuel" (furnace oil, kerosene etc.) rather than "petrol fuel" in that line.

[sridhar-v]

HI All! The correct way to settle the issue is to take a look at Total Cost of Ownership. TCO(on a monthly basis) Basic costs= Equated per month value of capital cost+pm fuel costs+pm maintenance costs+pm cost of value depreciation. My calculations show that you need to do atleast 3000 km per month to justify the difference in fuels costs of diesel vis a vis petrol. To the basic costs you need to add variable costs of the intangiables like:- Comfort of driving the particular model Performance style Relability premium Serviceability premium Imge/snob value. Things start becoming very difficult to calculate now and it then boils down to individual tastes. So to each his particular poison! Cheers Sridhar V

[vkumars]

The answer is 'I don't know'. Whether I choose petrol or diesel would depend on a number of factors.

I like both petrol and diesel cars - i.e., the free revving petrol engines and the fun to drive turbo diesels. Then what would be the main deciding factor ? Obviously, economy. Minor factors could be NVH levels and actual or perceived pollution levels. (A factor that currently influences the purchase is the equipment level available across diesel and petrol offerings from the same manufacturer. I am not taking that into consideration).

The economy comes in two ways. One natural and the other artificial. The natural part comes from diesel engines being more fuel efficient compared to petrol engines. This however is offset to a large extent by the higher maintenance cost of diesel engines.

The artificial part is provided by the government in two ways - by regulating the prices for diesel and by keeping a differential tax structure. It would be politically suicidal for any government to remove both. Since the transport and logistics industry depends on diesel, prices of all commodities would go through the roof.

If one part is removed, i.e., the diesel prices are deregulated, then also diesel would be attractive if lower tax rates are maintained. Of course, then the break-even arithmetic has to be redone to find whether purchasing diesels still makes economic sense. It might be that it would be economical only for very high mile crunchers.

The final decision would also depend on whether the government decides on any increase in excise duty for diesel vehicles for personal use.

To sum up, unless I know what the final figures would be, I would not be able to take a decision.

[Aroy]

If the purchase cost of vehicles are same or nearly so I will always go for diesel as it is more efficient and environment friendly. What is keeping me to petrol is the huge price difference for diesels, which really makes no sense at the lower end - nearly 1L premium at 5L Maruti Swift - Prices. At the 25L+ bracket the percentage is lower 2-3L at 25L. UK prices
BMW UK : BMW*3*Series*Saloon model range
BMW UK | BMW 5 Series | new Saloon | Prices and Specification

[vinya_jag]

I usually drive the Swift Diesel and Figo petrol.
I somehow like the Figo petrol more because of its revving nature. It easily hits the redline and is hungry always. Yes low fuel efficiency is one aspect, but keep that aside, I would always go for the petrol over diesel.

Fig P advantages-
Vrroom at traffic signals. I dont jump signals usually and hence am at the front most of the times. This is my opportunity to let the engine roar. Love the growl it makes.
Great for city driving
Cabin noise is almost zero on good road patches. Coupled with the excellent music system, its a heaven inside.

Figo P disadvantages-
Switch off the AC on highways when you want to overtake a rogue driver. This always happens on the Bangalore Mysore highway.
Takes more time from 60-100 if you hold on to 5th gear on inclines compared to Swift-D

Swift D advantages -
Easy to overtake at higher speeds, (say 80-110)
Sometimes fun to drive because of its turbo lag nature, the sudden rush of power is nice.

Swift D disadvantages -
Difficult in bumper to bumper traffic in city. By the time you get the power, the alto would have overtaken you.
No fun when it comes to the sound. Maintain correct gear all the time. Hitting redline is like hitting a standstill, no power whatsoever after 4000 rpm.
Noisy. Rear bench will not even hear whats playing

So for me, its petrol anytime handsdown

[alpha1]

Originally Posted by vina

Disclaimer: I would not buy a diesel car if petrol and diesel fuels cost the same, simply because the diesel car would still cost much more. If on the other hand the fuels cost the same and the difference in prices of the cars was small enough to be recovered by the diesels' better FE in a few years - I'll have to at least think very hard.

Presently I own a Figo TDCi


Having said that, I would say it is unfair to claim that petrol engines are better than diesels. In fact at a fundamental level, the diesel engines provide better combustion cycles, more flexibility as to what fuel can be used and a lot of other advantages. To my knowledge almost all large ICE engines are diesels - even the ones that use alternative fuel.

Also technology has benefited petrols also immensely: MPFI, VVT etc. help petrols too. In fact no amount of technology will ever help the basic petrol limitation of compression ratio, and a minimum fuel/air ratio.

Better = on performance terms.
If we go by thermodynamics - of course diesel cycle is better than Otto cycle.
The efficiency is higher - which makes the per litre fuel burnt in diesel engine making more bhp than petrol.
In other words - it gives better FE.

However, the performance that we widely talk about is bhp/engine mass.
This is where diesel engines fail - and have to have a TD/CRDI etc to come to par with petrol.

[vishnurp99]

love the diesel engines for the lowend torque that is available. The ford's tdci is a favourite of mine because of the lowend torque and driveability + the FE. Now I see a lot of people commenting that the petrol is better or the diesel is better. The fact is a person who is more used to the revvable nature of the petrol engine will get time to used to the diesel. Staying in the torque band/turbo range is slightly different from upshifting as soon as possible to a higher in a petrol right and revving away to glory.

if the diesel and petrol car prices are at par and the fuel prices are at par then i will go with the diesel purely because of FE and also because i am more comfortable with the torquey nature of the diesel engine.

[laxmanrk]

I would consider myself a Hardcore Diesel Head!

Even in the UK, where Diesel was more expensive than Petrol, I have only owned a diesel!

There are quite a few reasons for this, but primarily the following:

1) Efficiency - Diesels are much more efficient than petrols. I used to get about 50~60 mpg easily in the UK and all my friends and colleagues would be so much jealous of this.
2) Torque - low-end torque of diesels gives me so much of a high.

Finally, modern diesels have come a long way and even sports cars have started moving to diesel. (e.g. Audi). Its only a matter of time before we see a diesel Ferrari and a diesel Lamborghini

[.anshuman]

Most newer CRDI Diesel engines now come with wider powerband, most of these newer launches start pulling from less than 1500rpm. Some Examples-

• Toyota Fortuner- Starts pulling strongly from as low as 1300rpm. The low end is so strong that if i drive this SUV for a day, i find myself driving other cars in higher gears. Sure it does lack the punchy midrange of the Endeavour 3.0 but Power delivery is as linear as it can get for a Diesel.
  
• VW Vento- It lacks power near idle speeds but cross the 1400rpm marks it starts pulling seriously just like older diesels did after 1700rpm. The torque holds well upto 4500rpm.
  
• Laura/Jetta 110HP- The driveability of this engine is just unbelievable, the engine starts churning out peak torque from as low as 1500rpm and mated to wisely chosen ratios it makes for a very effortless and peppy drive.
  
• Laura/Yeti/Superb/A4 140hp- Driveability is as good as the engine mentioned above + it also has a very strong midrange and top end. I have not driven the 170hp version of this engine offered in Passat, online reviews say it's slightly more peaky but has more punch.
  
• Similarly the engine in BMW 320d/X1 too has good torque spread with very enthusiastic top end.

[vina]

Originally Posted by alpha1

Better = on performance terms.
If we go by thermodynamics - of course diesel cycle is better than Otto cycle.
The efficiency is higher - which makes the per litre fuel burnt in diesel engine making more bhp than petrol.
In other words - it gives better FE.

Great that we agree on the first part.

Originally Posted by alpha1

However, the performance that we widely talk about is bhp/engine mass.
This is where diesel engines fail - and have to have a TD/CRDI etc to come to par with petrol.

On this - you are right that the bhp/weight ratio wouldn't work for diesel. However I'm not sure that is the performance measure appropriate for usual car owners.

going deeper into this one, I'm sure you would agree that the performance measure much more relevant to bhp/engine weight would be :

(most used bhp)/(vehicle weight)

where (vehicle weight) is gross weight with typical load (one or two passengers with a little luggage for some people, and fully loaded vehicle for others) and (most used bhp) is the bhp that the vehicle produces in mid to low rpm regions (it'll be clear later why I'm excluding high rpm regions)

In petrol vs. diesel, the denominator of the equation hardly budges by 3% to 6% (hatches weigh about 50kg more vs. 1000kg total weight - difference for larger vehicles is even smaller in %)

As far as peak-bhp goes many cars (Figo, Fiesta, Logan ...) come with similar numbers between petrol and diesel - let us say another 5% difference. So (peak bhp)/(vehicle wieght) itself is usually different by only about 10% between petrol and diesel.

most used bhp is actually related to the bhp a user gets where he needs/wants is - and here (given how everyone raves about "diesel torque") diesel beats petrol very heavily for many (but surely not all) users. peak bhp numbers for most petrols are definitely higher but are invariably reached at higher rpm, while diesel's mid-rpm-range bhp is very close to its peak bhp.

I'm pretty sure if you use most-used-bhp/weight ratio, you'll understand why on this thread so many are so sentimental about their diesels.


Now coming to why I excluded high rpms from the discussion earlier:
My assumption is high power at high rpm is most useful on very high highway speeds (100kmph+) where the vehicle is mostly working against thermodynamic resistance. Weight plays a secondary role for such applications - frontal area of the vehicle and efficiency of streamlining are much more important. The other thing of importance is the raw bhp power available.

the performance criteria should be modified to:

(available bhp)/(air drag + tyre friction) with the air drag dominating heavily

In this region petrol beats diesel hands down, but not because of weight alone; diesel is bulky (will increase the frontal area) and in any case doesn't generate high rpm (and more importantly - it doesn't generate torque in a wide rpm range). Of course as anshuman mentioned this may not remain an advantage going forward.

bottomline is for a very large fraction of users, the power/weight ratio, if used appropriately actually favours the diesels.




AFAIK the only application where in power/weight ratio the weight of the engine is significant is fast flying jets - as we all know those use highly inefficient turbine engines - raw bhp is important as well as minimising the weight (fuel laden + engine) is important because higher weight = larger lift surfaces = higher drag and also because engine+fuel makes a big fraction of the overall vehicle weight